DURING CLB OUT FROM BFI; A C172 PLT INADVERTENTLY FLEW INTO SEA CLASS B AIRSPACE.

Narrative

I DEPARTED BOEING FIELD (BFI) IN SEATTLE; WA; AT APPROX XA30 LCL TIME ON AUG/SUN/01; INTENDING TO FLY TO ROCHE HARBOR IN THE SAN JUAN ISLANDS; WA. THE SKY WAS CLR AND VISIBILITY UNRESTR. MY FLT TRAINING FOR MY PVT CERTIFICATE WAS DONE AT BFI (CLASS D) IN 1999. MY LAST FLT FROM BFI HAD BEEN ON JUN/SAT/01. I HAD A CURRENT VFR AREA CHART FOR SEA CLASS B AND STUDIED IT PRIOR TO TKOF. INITIAL CLB WAS FROM BFI TO 1700 FT MSL. OUTSIDE OF BFI CLASS D AIRSPACE I INITIATED A CRUISE CLB; INTENDING TO CLB TO 4500 FT MSL FOR CRUISING. AT THIS TIME I HAD SWITCHED FREQ TO SEA DEP; INTENDING TO ASK FOR FLT FOLLOWING. I WAS ALSO SCANNING INTENTLY FOR OTHER TFC; AS TFC INBOUND TO BFI FROM THE N PASSES THROUGH THE SAME AREA. AT ABOUT XA45; APPROX 17 NM N OF SEA VOR (325 DEGS); MY PAX; A LICENSED PVT PLT WHO WAS MONITORING A PORTABLE GPS UNIT; STATED THAT WE WERE IN SEA CLASS B. MY ALTIMETER READ 3500 FT MSL. THE FLOOR OF CLASS B AIRSPACE AT THIS LOCATION WAS 3000 FT MSL. THIS WAS ENTIRELY UNINTENDED. I IMMEDIATELY BEGAN A DSCNT TO BELOW 3000 FT MSL. ACCORDING TO MY PAX'S GPS UNIT; WE DEPARTED CLASS B AIRSPACE WHEN THE ACFT REACHED 3100 FT MSL. THIS COULD HAVE BEEN AVOIDED IF I HAD TAKEN MORE TIME TO PLAN MY DEP FROM BFI AND THE SEA CLASS B AREA. I CONCENTRATED ON THE IMMEDIATE DEP FROM BFI; BUT NEEDED TO PLAN THE CLB TO CRUISING ALT AS WELL.